Conventionally, a spark plug for ignition is used in an internal combustion engine. A usual spark plug is configured by: an insulator which holds a center electrode in a tip end side of a shaft hole, and which holds a connecting terminal in a rear end side; a metal shell which surrounds and holds a trunk portion of the insulator; and a ground electrode in which one end is welded to the tip end of the metal shell, and the other end is opposed to the tip end of the center electrode to form a spark discharge gap.
The center electrode used in such a spark plug is formed by a highly refractory metal (for example, nickel). In order to further improve the refractoriness, an electrode is used in which a clad structure is configured with using a highly thermally conductive metal (for example, copper) as a core member, thereby enhancing the heat dissipation property. The center electrode having such a form is produced, for example, by extending a composite member in which a copper alloy is fitted into a cup formed by a nickel alloy, to a columnar shape by extrusion molding, and then applying a plastic working process on the extended member to obtain a desired electrode shape (for example, see Patent Reference 1).
Recently, in accordance with increase in output and reduction in fuel consumption of an automobile engine, it is requested to reduce the size and diameter of a spark plug from the viewpoint of the degree of freedom in design. In the case where a spark plug is produced while directly reducing the dimensions of components of a conventional spark plug, the clearance between a metal shell and an insulator is reduced, and there arises the possibility that a side spark occurs. A metal shell is restricted by the diameter of a screw for mounting to an engine, and also by the size of a ground electrode. Therefore, it is difficult to increase the inner diameter of the metal shell. When the thickness of the insulator is reduced in order to ensure the clearance, there is the possibility that the strength is lowered, or that the insulation is insufficient. When the outer diameter of the center electrode is thinned while the outer diameter of the insulator is reduced and the clearance with respect to the metal shell is ensured, consequently, the thickness of the insulator is not reduced and the strength can be maintained.
Patent Reference 1: JP-A-8-213150